1. Field of the Invention
The present invention relates generally to systems and methods for half-toning images, and more particularly, to a method and system for watermarking images during the halftone process.
2. Description of the Prior Art
Watermarking of digital images is a process in which information is added to or embedded in the digital images. The embedded information should be robust to non-destructive image processing procedures, and the embedding process should not adversely affect the quality of the digital image. In other words, a good digital watermark cannot be removed from the image without significantly degrading the image, and should not be easily noticeable.
In conventional watermarking processes, the watermarking is accomplished by taking a digital image, such as a continuous-tone image that is suitable for display, and adding the watermark to obtain a watermarked continuous-tone image. The watermark can be added by performing a mathematical transform operation (e.g., a Fourier Transform operation) on the continuous-tone image, adding a watermark to the transformed image, and then performing an inverse transform operation to obtain the watermarked continuous-tone image. This conventional watermarking process can be utilized to unauthorized digital copies of an image. For example, if a digital image having a watermark therein is found uploaded on an Internet Web page, but it is determined that the image is not authorized (i.e., the owner of the image has not granted permission to the Web site operator to post the image), appropriate steps can be taken to remedy the situation.
Unfortunately, the prior art watermarking techniques have generally been limited to digital images and have not been extended to hard copy images, especially those images that have been rendered by a halftone process.
As the print quality of printers that employ a halftone process increases, there is an increasing threat that these printers be utilized in illegal or fraudulent operations. For example, one such operation is the printing of counterfeit currency and other counterfeit prints (e.g., hard copies of copyrighted artwork and images) by using these half-tone printers. As can be appreciated, there are numerous applications where it would be desirable to be able to use an offending print or fraudulent print (e.g., a document that violates the copyrights of a third party or counterfeit currency) to uniquely identify the source that generated the print.
Unfortunately, the prior art methods of watermarking cannot be used to identify a printer from which a particular print is printed since the prior art watermarking is performed independent of and separate from the half-toning process.
It is noted that adding a watermark into the halftone process is not a simple, straightforward, or trivial task. For example, if one simply attempted to print using a half-tone process a digital image, which is watermarked in the process described above, several serious problems would occur. First, the watermark generated by prior art approaches is not robust to printing and scanning. In fact, when a digital image that has a watermark embedded therein by a prior art watermarking process is half-toned, the halftone process injects severe noise to the watermarked image. This noise increases the likelihood that the watermark cannot be recovered accurately. Second, there is also the noise due to the printing and scanner process that is problematic for the watermark generated by the prior art.
Accordingly, there still remains a need for a technique for watermarking images during the half-tone process that overcomes the disadvantages set forth above.
According to one embodiment of the present invention, a method for embedding a watermark during the half-tone process, generating a hard copy of the watermarked half-tone image, receiving a scanned image of the hard copy, and recovering the watermark from the scanned image. Adding or encoding the watermark includes receiving a continuous tone image, receiving the watermark information to be embedded, and receiving the locations in the image where the watermark is to be imposed or the location determination criteria. Based on these inputs, a watermarked half-tone image is generated during the half-tone process. Recovery of the watermark (or decoding the watermark) involves scanning a hard copy that includes a half-tone image with a watermark. Next, the watermark locations of the watermark or the watermark location determination criteria are received. Based on these inputs, the watermark is recovered.
In another embodiment of the present invention a watermarking system is provided. The watermarking system includes a watermarking module including a first input for receiving a continuous half-tone image, a second input for receiving watermark information, and a third input for receiving watermark locations. The watermarking module generates a watermarked half-tone image that has the watermark imposed at the watermark locations in the half-tone image. The watermarking system can also include a print engine for receiving the watermarked half-tone image and based thereon for generating a hard copy of that image.
The watermarking system also includes a watermark recovery module that has a first input for receiving a scanned image, which can be provided by a scanner, and a second input for receiving watermark locations. The watermark recovery module recovers the watermark from the scanned image.